1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device such as a flash memory and a data-write method thereof.
2. Description of the Related Art
In a flash memory, data is stored by changing the threshold voltage of the floating gate of a memory-cell transistor by changing the amount of charge stored therein through erase and write operations. For example, data “1” is stored by releasing electrons from the floating gate, thereby allowing the threshold to be negative and data “0” is stored by injecting electrons.
In a NAND flash memory, data 1 or 0 is distinguished by supplying, for example, 0 V, to a word line of the memory cell selected in a readout operation. A plurality of unselected memory cells is connected in series to the selected memory cell. Therefore, for example, 4.5 V, is supplied to the word lines of the unselected memory cells to bring the unselected memory cells into an electrically conductive state independently of the data stored therein. Therefore, the threshold of the memory cell to be written during a write operation must be sufficiently controlled so that it falls within the range of 0 to 4.5 V.
Therefore, to control the threshold voltage of a memory cell during the write operation, the commonly used data write method employs a “step-up write voltage”. Such a data write method is described in Fast and Accurate Programming Method for Multi-level NAND EEPROMs, pp 129-130, Digest of 1995 Symposium on VLSI Technology.
The data write method using the “step-up write voltage” employs the following characteristic feature: when the write voltage to be supplied to a memory cell is increased at a predetermined rate (e.g., 0.2 per 10 μsec), the threshold voltage increases at the same rate (e.g., 0.2 per 10 μsec). Further in this method, the threshold voltage of each memory cell is detected at intervals of 10 μsec and when the threshold voltage reaches a predetermined write verify voltage, the write operation is inhibited. In this manner, the threshold voltage is controlled so that it falls within a margin 0.2V above the write-verify voltage.
In the method mentioned above, the write operation is controlled by using the feature that the threshold voltage increases at a constant rate. Therefore, before the verify operation is initiated, a so-called pre-write operation is performed by setting the initial write voltage to a sufficiently low value in order to increase the threshold voltage at the constant rate in advance.